Is the +1 cumulative per attack on the same target (draining more and more blood)?
No. Its just a straight +1 bonus to all further attacks. Its meant to show that you're already latched on so its easier to hurt the target, not really to show how much you drank.
Does the drinked blood help recover from any feeding dependency negative effect (failure recovery)?
Not unless you as GM decide it does. but the extra recovery scene will help in recovering from any consequences he's taken from feeding failure.
TASTE OF DEATH
If you kill a victim from feeding...
Does it mean killing a victim using the DRINK BLOOD effect?
Yeah. Its just that the Emotional Vampire power has the same trapping and they probably didn't want to have to write it twice by being too specific.
If you ever benefit from this effect, you are essentially moving from Infected mortal to full fledged Red Court right?
Yup. If a Red Court Infected ever uses the taste of death trapping, he must immediately upgrade to the full Red Court Vampire template (and almost certainly lose his character as thats a refresh 11 template).